Radar level gauging using frequency modulated pulsed wave

ABSTRACT

A method for measurement of a distance to a surface of a product kept in a tank method comprises transmitting a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, the pulse train has an average duty cycle of less than 50 percent, each pulse has a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, greater than 5% of an average center frequency. The method further comprises correlating actual phase properties of received pulses with expected phase properties to provide an updated estimation of the distance. 
     The present invention is based on transmitting a set of carrier wave pulses, each having a distinct frequency selected within a frequency range. The method is therefore referred to as a Frequency Modulated Pulsed Wave (FMPW).

FIELD OF THE INVENTION

The present invention relates to a radar level gauge using electromagnetic waves to determine a distance to a surface of a product in a tank.

BACKGROUND OF THE INVENTION

Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 25-27 GHz, or 9.5-11 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. Due to the linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.

More recently, the FMCW principle has been improved, and today typically involves transmitting not a continuous sweep but a signal with stepped frequency but practically constant amplitude. An example of such a stepped FMCW is disclosed in U.S. Pat. No. 5,406,842. For a number of discrete frequencies a phase shift is determined, and based on a set of such phase shifts an IF signal is obtained, having the same properties as the continuous IF signal described above. In principle, this requires a number of frequencies, N, greater than a number stipulated by the sampling theorem. The distance to the reflecting surface is then determined using the frequency of the IF signal in a similar way as in a conventional FMCW system. Typical values can be 200-300 IF periods at 30 m distance divided in 1000-1500 steps.

Although highly accurate, FMCW systems are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell).

GENERAL DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improved method for radar level gauging, with lower power requirements than the conventional FMCW method.

According to a first aspect of the present invention, this and other objects are achieved by a method for measurement of a distance to a surface of a product kept in a tank.

The method comprises transmitting low power electromagnetic transmit signals towards the surface, receiving electromagnetic return signals reflected at the surface, determining the distance based on an initially estimated distance and a relationship between the transmit signals and the return signals. The transmit signals are formed as a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, the pulse train has an average duty cycle of less than 50 percent, each pulse has a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, greater than 5% of an average center frequency. The method further comprises determining actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, determining, based on the initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse, and correlating the actual phase properties with the expected phase properties to provide an updated estimation of the distance.

The present invention is based on transmitting a set of carrier wave pulses, each having a distinct frequency selected within a frequency range. The method is therefore referred to as a Frequency Modulated Pulsed Wave (FMPW).

Unlike the method referred to above as stepped FMCW, the number of different carrier wave frequencies in a measurement cycle is insufficient to provide a continuous IF signal, or even an approximation of the IF frequency used in a conventional FMCW system. Instead, the small set of frequencies is chosen according to a specified frequency scheme, and a phase shift in the received pulse is determined for each frequency. The set of phase shifts allow a determination of a change compared to a previously recorded distance to the reflecting surface. In most applications the user requires an updating rate in the order of once per second and then the level change between two measurements is small.

As the pulses are emitted and processed distinctly, and independently of each other, the duty cycle of the transmit signal can be reduced, and is less than 50%. In some embodiments of the invention it is significantly less, and may be 5% or even 1% or lower. This means that individual pulses may be emitted with a relatively high power, without increasing the average power of the measurement cycle. This makes the present invention particularly useful in situations where power is scarce, such as in a field unit powered by an industrial current loop (e.g. 4-20 mA loop) or in a battery power (or solar powered, etc) field unit.

The correlating can include determining a distance offset based on the actual phase properties and said expected phase properties for each carrier wave frequency, and determining the updated estimation of the distance based on the initially estimated distance and the distance offset.

Alternatively, the correlating can include determining a relationship between actual phase and carrier wave frequency. Such a relationship may be expressed as a slope of a line in a diagram, which slope indicates the distance.

The order of the pulses with difference frequencies is not critical for the determination of the distance, and the pulses may for example be transmitted in ascending or descending order, or any mixed order.

The frequency scheme may be designed in various ways, and may include dividing an overall frequency range (e.g. in the order of GHz) into a number of subranges, and defining a subset of discrete frequencies for each subrange. The schedule may further include randomly selecting one frequency each from each frequency subset.

According to one embodiment of the invention, the schedule is adaptive, i.e. it may vary depending on the current measuring situation. For example, it may be advantageous to increase the number of pulses when more difficult measuring conditions are at hand. Such difficult measuring conditions may include a disturbed or quickly moving surface, and may present themselves as an increased uncertainty in the measurement result (quantified e.g. as a variance or standard deviation).

In this case, the schedule may include several accuracy levels, each including a different number of frequencies. When a measurement result acquired using one accuracy level becomes too uncertain, the accuracy level is increased, and a larger number of pulses with different frequencies are used in the next cycle.

Conventional FMCW with a linear or stepped sweep converts the set of echoes from various distances in the tank to a “tank spectrum”. The interesting echo may be filtered out to decrease disturbing echoes from other parts of the tank. This is not possible with the simplest type of MFPW according to the present invention. To improve the situation it may be advantageous to introduce a frequency modulation of the carrier wave, and to mix the received signal with the modulation frequency, in order to provide dependence on the distance. This dependence can be used to eliminate undesired echoes by low pass filtering.

The transmitter signal is preferably frequency modulated by a suitable frequency f_(m) (several complete periods within each pulse), providing a variation of the carrier frequency within a frequency band δf in the order of MHz. With such modulation, two advantages are available:

-   -   1. The signal can be received around f_(m) or a multiple of         f_(m) (instead of DC) which will make it less sensitive for 1/f         noise and transient-like disturbances.     -   2. A distance dependence is introduced, which might be         advantageous for instance to limit the echo competition to the         nearest ±1 m etc.

A second aspect of the present invention relates to a system for FMPW, comprising a transceiver for transmitting low power electromagnetic transmit signals and receiving electromagnetic return signals reflected at the surface, and processing circuitry for determining the distance based on an initially estimated distance and a relationship between the transmit signals and the return signals. The transceiver is arranged to form the transmit signals as a pulse train of distinct carrier wave pulses having a duration greater than 1 microseconds and shorter than 100 milliseconds, the pulse train having an average duty cycle of less than 50 percent, each pulse having a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, greater than 5% of an average center frequency. The processing circuitry is arranged to determine actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, determine, based on the initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse, and correlate the actual phase properties with the expected phase properties to provide an updated estimation of the distance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail with reference to the appended drawings, showing currently preferred embodiments of the invention.

FIG. 1 is a schematic block diagram of a radar level gauge mounted on a tank.

FIG. 2 is a more detailed block diagram of the transceiver in FIG. 1, according to a first embodiment of the present invention.

FIG. 3 is a more detailed block diagram of the transceiver in FIG. 1, according to a second embodiment of the present invention.

FIG. 4 is a diagram showing phase versus transmitted frequencies.

FIG. 5 is a flow chart of a method according to an embodiment of the present invention.

FIG. 6 illustrates frequency modulation of a carrier wave.

FIG. 7 shows the first five harmonics for sinusoidal modulation.

FIG. 8 shows Fourier coefficients for even tones for triangular modulation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the present description, embodiments of the present invention are mainly described with reference to a radar level gauge system having a free propagating antenna for radiating and capturing electromagnetic signals. It should be noted that this by no means limits the scope of the invention, which is equally applicable to other signal propagating devices, including other free propagating antennas such as a rod antenna, a patch antenna, a fixed or movable parabolic antenna or a conical antenna, and wave guides, such as a still pipe, a transmission line or a probe, such as a single-line probe (including a so-called Goubau probe), a twin-line probe or a coaxial probe.

FIG. 1 schematically illustrates a radar level gauge system 1 according to an embodiment of the present invention, comprising a measurement electronics unit 2, and a signal propagating device, here a horn antenna 3. The radar level gauge system 1 is provided on a tank 5, which is partly filled with a product 6 to be gauged. In the case illustrated in FIG. 1, the product 6 is a solid, such as grain or plastic pellets, which is well-known to represent a difficult application requiring a relatively high measurement sensitivity of the radar level gauge system. By analyzing transmitted signals S_(T) being radiated by the antenna 3 towards the surface 7 of the product 6, and echo signals S_(R) traveling back from the surface 7, the measurement electronics unit 2 can determine the distance between a reference position and the surface 7 of the product 6, whereby the filling level L can be deduced. It should be noted that, although a tank 5 containing a single product 6 is discussed herein, the distance to any material interface present in the tank 5 can be measured in a similar manner.

As is schematically illustrated in FIG. 1, the electronics unit 2 comprises a transceiver 10 for transmitting and receiving electromagnetic signals, which is connected to the antenna 3 via a wave guide 9. The unit 2 further comprises a processing circuitry 11, which is connected to the transceiver 10 for control of the transceiver and processing of signals received by the transceiver to determine the filling level of the product 6 in the tank 5. The processing circuitry 11 is also connected to a memory 12, storing any software required for the operation of the gauge 1, and also providing RAM used during operation.

The processing circuitry 11 is further connectable to external communication lines 13 for analog and/or digital communication via an interface 14. As an example, the communication between the communication interface 14 and an external control station (not shown) can be provided by a two-wire interface, which has a combined function of both transmitting the measurement result to the control station and receiving power for operation of the gauge 1. The current in the lines is regulated in accordance with a measurement result determined by the processing circuitry 11.

Alternatively, the gauge may communicate wirelessly with the control station using e.g. a Wireless HART protocol, and use a local power supply with batteries or other means of scavenging energy for autonomous operation.

Although being shown as separate blocks in FIG. 1, several of the transceiver 10, the processing circuitry 11 and the interface 12 may be provided on the same circuit board.

FIG. 2 schematically shows an exemplary transceiver 10 suitable for implementing an embodiment of the present invention.

A signal generator 20 is arranged to generate carrier wave pulses with a well defined and accurately controlled carrier frequency. The accuracy deviation should be less than 1/1000, and preferably less than 1/10000 or better. This may be achieved with a feed back control system, and the signal generator may for example be a phase locked loop, PLL.

In the illustrated example discussed herein, the pulse duration will be assumed to be about 2 ms, but significantly shorter pulses, e.g. in the order of μs are also possible. It should be noted that in a tank gauging context, pulses of a duration in the order μs or ms, as mentioned here, are much too long to enable pulsed distance measurement with time domain reflectometry, i.e. to receive a reflected pulse and determine its time of flight. The pulses according to the present invention are thus longer than pulses in conventional pulsed radar level gauges, where a typical pulse duration is in the order of ns.

The average power of the pulses can be in the nW or μW area. However, the duty cycle, i.e. the relationship between the pulses and the intervals there between, should be limited to less than 50%, and is preferably significantly lower, e.g. 5% or even 1% or less. Compared to conventional FMCW, this means that the pulses may be emitted with a relatively high power, without increasing the average power of the measurement cycle.

The pulses are typically radar pulses, with a carrier wave frequency in the GHz area, e.g. in the range 9,5-11 GHz or 24-26 GHz. The duration of the pulses, the duty cycle, and the frequency of the PLL are controlled by the processor 11 (see FIG. 1).

A circulator, or a power divider such as a Wilkinson Power Divider (WPD) 22, is provided to direct the signal from the signal generator 20 to the antenna 3 in the tank 5 and to direct reflected signals from the antenna 3 to a receiver section of the transceiver 10.

The receiver section comprises two channels, an in-phase channel (I) and a quadrature channel (Q), each comprising a mixer 23 a, 23 b for homodyne mixing of the received signal. The first mixer 23 a is provided with the transmitted pulses directly from the PLL 20 (I-channel). The second mixer 23 b is provided with the transmitted pulses from the PLL 20 via a 90 degrees phase shifter 24 (Q-channel).

Each channel comprises a low pass filter 25 a, 25 b and an amplifier 26 a, 26 b. The filters and amplifiers improve the signal-to-noise ratio of the radar level gauge system.

The low pass filters 25 a, 25 b preferably have a bandwidth corresponding to the inverse of the pulse width. In other words, if the duration of the pulse is 2 ms, a suitbale filter will be 500 Hz wide. Longer pulses can thus be used to increase the sensitivity (smaller bandwidth) at the expense of the average power. However, if a low duty cycle is desired, in order to achieve a low average power consumption, the pulses should be kept reasonable short, and the bandwidth of the low pass filters 25 a, 25 b must then be wider (with less sensitivity) than a conventional FMCW.

The signals are then supplied to two integrators 27 a, 27 b, for integrating the signals over the length of the pulse. Finally two A/D-converters 28 a, 28 b are arranged to A/D convert the integrated value, and the digital output is supplied to the processor 11 for storage and processing.

The embodiment in FIG. 2 may be sensitive to disturbing echoes, as it is not possible to filter out a relevant portion of the reflected signal. In other words, this embodiment is best suited for measuring conditions where the surface reflection dominates any other reflections.

FIG. 3 shows a transceiver according to a further embodiment of the present invention, introducing a modulation of the carrier wave. This modulation is not necessary for the general principle of the present invention, but may serve to improve reliability, as will be explained below.

Referring to FIG. 3, the PLL 20 is connected to a modulator 30, arranged to frequency modulate the pulses from the PLL 20 with a modulating frequency f_(m). The frequency variation of the modulated carrier wave is shown in FIG. 6. The modulation frequency should be chosen with respect to the pulse duration to ensure that a complete number of modulation periods (P_(mod)) are included in each pulse. In the present example, where the pulse length is in the order of ms, the modulating frequency should be in the order of kHz, and may for example be 10 kHz. For a μs pulse duration, the modulating frequency should be in the order of MHz.

The modulation has an amplitude selected to introduce a variation of the carrier wave frequency δf, which is significantly smaller than the predetermined frequency range of the measurement. As mentioned, the predetermined range of carrier wave frequencies used in the frequency scheme may be in the order of 1 GHz, and the modulation 8 f can then suitably be in the order of MHz.

In this embodiment, the output from the mixers 23 a, 23 b will not be DC level signals, but instead be IF (intermediate frequency) signals including the modulation frequency f_(m) and harmonics thereof. The mixers 23 a, 23 b are connected to band pass filters 31 a, 31 b, arranged to filter out the desired frequency content. As discussed below, this is typically one of the first harmonics of f_(m). The filtered signals are amplified by amplifiers 32 a, 32 b.

Each channel then comprises a second set of mixers 33 a, 33 b, which also receive as input the modulation frequency f_(m) from modulator 30, via a multiplier 34. The multiplier 34 is adapted to provide a multiple N of the modulation frequency f_(m), corresponding to the harmonic that has been filtered out by filters 31 a, 31 b. For example, if the first harmonic is selected, N=2.

The output from the mixers 33 a, 33 b will be DC level signals, which are fed to low pass filters 25 a, 25 b, amplifiers 26 a, 26 b, integrators 27 a, 27 b and A/D converters 28 a, 28 b, corresponding to the elements in FIG. 2.

Depending on the type of frequency modulation introduced by the modulator 30 (sinusoidal, triangular etc.) the IF signal output by mixers 25 a, 25 b will have a distance dependence and will be spread over different harmonics. If the transmitted signal has a sinusoidal modulation with modulation width δf, the received signal after the homodyne mixing can be described by Bessel functions as harmonics of f_(m) as illustrated in FIG. 5.

The argument in the Bessel functions is X=2πhαf/c, where h is the distance to the reflecting surface and c is the speed of light in vacuum. With increasing distance h (and thus increasing X) the received signal is spread over more harmonics. If, for example, the modulation width δf is 15 MHz and the distance h is 10 m, the argument X will be ˜2.0 and most received signal will occur in f_(m). The distance dependence of J1(X) (for X smaller than 2) has the same feature as the high-pass filtering in conventional FMCW-systems, i.e. to reduce the dynamic range ˜20 dB by having an amplitude distance dependence counteracting the 1/h-dependence. If a fixed δf is used it will be limited to 8-10 MHz to measure up to 20-25 m without losing too much amplitude by spreading power to too many other harmonics.

It is also possible to pick out the second tone, 2f_(m), which is more efficient to suppress close echoes and allows 12-15 MHz modulating width in applications to 20-25 m. Note that in some cases the filters 31 a, 31 b are determined by hardware, and not variable. Instead, it is the modulator frequency f_(m), modulating width δf, and factor N that are varied in order to achieve the desired filtering.

An alternative to sinusoidal modulation is triangular frequency modulation which has different properties, and may be more efficient than sinusoidal modulation.

The resulting Fourier coefficients for the even harmonics (2f_(m), 4f_(m), etc) at triangular modulation are shown in FIG. 6. The coefficients for 1, 3 etc. are very similar to those for 2, 4 etc. The peaks are all close to 1 so if the closest harmonic is filtered out nearly all signal power is used. The argument X is the same as above, so a wider of may be used in the case of triangular modulation. At 50 MHz modulation width the full axis in the diagram above (0<x<20) corresponds to 0-30 m distance. With 50 MHz modulation width the spectral density can be reduced 17 dB without reducing the received signal. This reduction will remain also for long pulses.

Just as for sinusoidal modulation, the filtering can be changed between different harmonics, either by variable (digital) filters, or by varying f_(m) to fit the suitable harmonic to a fixed filter. In this way the function will be rather close the conventional FMCW-system using a filter to pick out the signal to accurate differential phase calculation.

By selecting a plurality of harmonics a simple distance estimation may further be achieved. Basically, each harmonics represents a given distance range. By determining the received power in several such ranges, and correlating them to each other, the distance may be estimated. Such distance estimation is known in the art, and may be useful in particular under noisy conditions, such as when the surface is turbulent.

The operation of the gauge according to an embodiment of the invention will now be disclosed with reference to FIGS. 4 and 5. The curve 41 in FIG. 4 corresponds to a distance measurement completed during an initialization phase (step S1 in FIG. 5). This measurement can have been accomplished by a conventional radar level gauging method, such as FMCW (continuous or stepped) or TDR. It may also have been accomplished by any other detection method available, including a differential pressure detection and an induction detection.

Regardless of how the measurement has been completed, the detected distance can be converted into phase differences any given frequency. The curve 41 in FIG. 4 corresponds to a set of such phase differences.

A measurement cycle according to an embodiment of the present invention can now be performed. First, in step S2, a frequency scheme is determined according to a predefined procedure. In one embodiment, this procedure includes selecting one frequency from each of N subsets of frequencies in the sweep range. The sweep range may be in the order of 10% of the operating frequency, and may be e.g. between 25 and 27 GHz, or between 9,5 and 11 GHz. The number of frequency subsets N may preferably be determined based on the conditions prevalent in the tank. During advantageous conditions (slowly moving surface, small noise, no or weak disturbing echoes) a small number of frequencies may be sufficient, and N may be less than 10. During intermediately difficult conditions, N may be in the range 10-100, and for even more difficult conditions, N may be greater than 100. As comparison, a conventional stepped FMCW measurement typically includes around 2000 frequencies.

The N frequencies may be selected randomly from the N subsets. Further, the frequencies need not be in magnitude order, so the pulse frequency may alternate up and down.

In the illustrated example, the scheme comprises five different frequencies, spread out over a 2 GHz range.

Then, in step S3-S5, the sweep is performed with the selected frequencies in the selected order. For each frequency, a pulse of a determined duration, e.g. 2 ms, is generated by the PLL 20 and transmitted as signals ST into the tank by the antenna 3 (step S3).

The transmitted electromagnetic signals ST are reflected at impedance transitions in the tank 5, including the surface 7 of the product 6 contained in the tank 5, and are returned to the transceiver 10 as echo signals SR by the antenna 3. The reflected signal SR is received (step S4) by the two channels (I and Q) in the receiver side of the transceiver 10.

Then, in step S5, a phase difference between the transmitted pulse and the reflected pulse is determined by the processor 11 using the output from the two channels and conventional I/Q processing.

The various phase shifts are recorded in memory 13 (step S6), and steps S3-S6 are repeated for all frequencies in the scheme.

In step S7, each phase difference value (which is in the range 0-2 pi) is correlated to an expected phase difference value, calculated based on the detected distance. In FIG. 4, the various phase differences 42 a-f have been plotted just above the line 41.

The difference between the expected phase and the actual, detected phase corresponds to a distance offset. In principle, one such offset, resulting from a single frequency, is sufficient to provide an updated distance detection. However, the uncertainty in such a detection will normally be too significant to provide satisfactory reliability, and some kind of statistical analysis will normally be required.

One approach is to determine an average distance offset defined by the average difference between the actual, detected phase compared to the expected phase.

Another approach is to identify a straight line (dotted line 43) which is correlated to the five phase shift values (e.g. in the least square sense). The slope of this line can be converted to a detected distance.

Note that the number of pulses used (number of different frequencies) is too small to provide a uniquely defined distance measurement by themselves. In the approaches above, knowledge of a previously detected distance (the straight line 41) is therefore used to provide expected phase values, and eventually a distance estimation. If a previous detection is not available, or deemed desirable, it may be possible to determine an estimated distance by simulation.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the selection of frequencies may be performed in many alternative ways. Further, the details of the transceiver circuitry may be modified in a number of ways, without departing from the inventive concept. 

What is claimed is:
 1. A method of providing low power Multiple Frequency Pulsed Wave measurement of a distance to a surface of a product kept in a tank, said method comprising: transmitting low power electromagnetic transmit signals towards said surface, receiving electromagnetic return signals reflected at said surface, determining said distance based on an initially estimated distance and a relationship between said transmit signals and said return signals, said transmit signals being formed as a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, said pulse train having an average duty cycle of less than 50 percent, each pulse having a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, said predetermined frequency range being greater than 5% of an average center frequency, determining actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, determining, based on said initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse, and correlating said actual phase properties with said expected phase properties to provide an updated estimation of said distance.
 2. The method according to claim 1, wherein said initially estimated distance is acquired by an initial level measurement.
 3. The method according to claim 1, wherein said correlating includes: determining a distance offset of said distance based on said actual phase properties and said expected phase properties, and determining said updated estimation of said distance based on said initially estimated distance and said distance offset.
 4. The method according to claim 1, wherein said correlating includes determining a relationship between actual phase and carrier wave frequency.
 5. The method according to claim 1, further comprising: homodyne mixing the return signal with the transmit signal in a first channel (I-channel) and homodyne mixing the return signal with a 90° phase shifted transmit signal in a second channel (Q-channel), to provide two amplitude values, and calculating said phase properties of each distinct received pulse based on said amplitude values.
 6. The method according to claim 1, further comprising: determining a variance of said updated estimation, and updating said frequency scheme to include a greater number of frequencies if said variance exceeds a predetermined variance threshold.
 7. The method according to claim 1, further comprising frequency modulating each carrier wave pulse with a modulation frequency.
 8. The method according to claim 7, further comprising: homodyne mixing the return signal with the transmit signal in a first channel (I-channel) and homodyne mixing the return signal with a 90° phase shifted transmit signal in a second channel (Q-channel), to provide two IF (intermediate frequency) signals, filtering said IF signals to provide a filtered signal corresponding to a selected harmonic of said modulation frequency, mixing the filtered signals of each channel with said selected harmonic of said modulation frequency, to provide two amplitude values (I and Q), and calculating said phase properties of each distinct received pulse based on said amplitude values.
 9. The method according to claim 7, wherein the frequency modulation is one of sinusoidal and triangular.
 10. The method according to claim 7, wherein the modulation frequency is in the range 10kHz-100kHz.
 11. The method according to claim 7, wherein the modulation introduces a variation of the carrier frequency in the range 1MHz-100MHz.
 12. The method according to claim 1, wherein said frequency scheme is determined by: defining N subsets of frequencies within the predetermined frequency range and randomly selecting one frequency from each subset.
 13. The method according to claim 1, wherein each carrier wave frequency is controlled in a fed-back control loop providing a frequency accuracy deviation less than 1/1000.
 14. The method according to claim 1, wherein said predetermined frequency range is greater than 10% of an average center frequency.
 15. The method according to claim 1, wherein said duty cycle is less than 5%.
 16. A Multiple Frequency Pulsed Wave radar level gauging system, for detecting a distance to a surface of a product in a tank, said system comprising: a transceiver for transmitting low power electromagnetic transmit signals and receiving electromagnetic return signals reflected at said surface, processing circuitry for determining said distance based on an initially estimated distance and a relationship between said transmit signals and said return signals, said transceiver being arranged to form said transmit signals as a pulse train of distinct carrier wave pulses having a duration greater than 1 microseconds and shorter than 100 milliseconds, said pulse train having an average duty cycle of less than 50 percent, each pulse having a defined center frequency, selected according to a frequency scheme within a predetermined frequency range, said predetermined frequency range being greater than 5% of an average center frequency, said processing circuitry being arranged to: determine actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, determine, based on said initially estimated distance, expected phase properties of each received distinct pulse in relation to each corresponding transmitted distinct pulse, and correlate said actual phase properties with said expected phase properties to provide an updated estimation of said distance.
 17. The system according to claim 16, further comprising: a first mixer for homodyne mixing the return signal with the transmit signal in a first channel (I-channel) to provide a first amplitude value, and a second mixer for homodyne mixing the return signal with a 90° phase shifted transmit signal in a second channel (Q-channel) to provide a second amplitude value, wherein said processing circuitry is further arranged to calculate said phase properties of each distinct received pulse based on said first and second amplitude values.
 18. The system according to claim 16, further comprising a modulator for frequency modulating each carrier wave pulse with a modulation frequency.
 19. The system according to claim 18, further comprising: a first mixer for homodyne mixing the return signal with the transmit signal in a first channel (I-channel) to provide a first IF (intermediate frequency) signal, and a second mixer for homodyne mixing the return signal with a 90° phase shifted transmit signal in a second channel (Q-channel) to provide a second IF (intermediate frequency) signal, filters for filtering said first and second IF signals to provide a filtered signal corresponding to a selected harmonic of said modulation frequency, and mixers for mixing the filtered signal of each channel with said selected harmonic of said modulation frequency, to provide two amplitude values (I and Q), and wherein said processing circuitry is further arranged to calculate said phase properties of each distinct received pulse based on said amplitude values.
 20. The system according to claim 18, wherein the modulator is arranged to provide one of sinusoidal and triangular frequency modulation.
 21. The system according to claim 18, wherein the modulator is arranged to provide a modulation frequency in the range 10kHz-100kHz.
 22. The system according to claim 18, wherein the modulator is arranged to introduce a variation of the carrier frequency in the range 1MHz-100MHz.
 23. The system according to claim 16, further comprising a fed-back control loop for controlling each carrier wave frequency to provide a frequency accuracy deviation less than 1/1000.
 24. The system according to claim 16, wherein said predetermined frequency range is greater than 10% of an average center frequency.
 25. The system according to claim 16, wherein said duty cycle is less than 5%. 